Muscular dystrophies (MDs) are a group of genetic diseases. The group is characterized by progressive weakness and degeneration of the skeletal muscles that control movement. Some forms of MD develop in infancy or childhood, while others may not appear until middle age or later. The disorders differ in terms of the distribution and extent of muscle weakness (some forms of MD also affect cardiac muscle), the age of onset, the rate of progression, and the pattern of inheritance.
Facioscapulohumeral muscular dystrophy (FSHD) is a complex autosomal dominant disorder characterized by progressive and asymmetric weakness of facial, shoulder and limb muscles. Symptoms typically arise in adulthood with most patients showing clinical features before age thirty. About five percent of patients develop symptoms as infants or juveniles and these are generally more severely affected. Clinical presentation can vary from mild (some limited muscle weakness) to severe (wheelchair dependence). Historically, FSHD was classified as the third most common MD, affecting one in 20,000 individuals worldwide. However, recent data indicate FSHD is the most common MD in Europe, suggesting its worldwide incidence may be underestimated.
Typical FSHD cases (FSHD1A, heretofore referred to as FSHD) are linked to heterozygous chromosomal deletions that decrease the copy number of 3.3 kilobase (kb) D4Z4 repeats on human chromosome 4q35. Simplistically, normal individuals have 11-100 tandemly-repeated D4Z4 copies on both 4q35 alleles, while patients with FSHD have one normal and one contracted allele containing 1-10 repeats. In addition FSHD-associated D4Z4 contractions must occur on specific disease-permissive chromosome 4q35 backgrounds. Importantly, no genes are completely lost or structurally mutated as a result of FSHD-associated deletions. Thus, although the disease was formally classified in 1954, and the primary genetic defect identified in 1992, the pathogenic mechanisms remain unresolved.
In leading FSHD pathogenesis models, D4Z4 contractions are proposed to cause epigenic changes that permit expression of genes with myopathic potential. As a result, aberrant over-expression of otherwise silent or near-silent genes may ultimately cause MD. This model is consistent with data showing normal 4q35 D4Z4 repeats have heterochromatin characteristics, while FSHD-linked D4Z4 repeats contain marks more indicative of actively transcribed euchromatin. These transcription-permissive epigenetic changes, coupled with the observation that complete monosomic D4Z4 deletions (i.e., zero repeats) do not cause FSHD, support the hypothesis that D4Z4 repeats harbor potentially myopathic open reading frames (ORFs), which are abnormally expressed in FSHD muscles. This notion was initially considered in 1994, when a D4Z4-localized ORF, called DUX4, was first identified. However, the locus had some characteristics of an unexpressed pseudogene and DUX4 was therefore summarily dismissed as an FSHD candidate. For many years thereafter, the search for FSHD-related genes was mainly focused outside the D4Z4 repeats, and although some intriguing candidates emerged from these studies, no single gene has been conclusively linked to FSHD development. This slow progress led to the re-emergence of DUX4 as an FSHD candidate in 2007. Even as of 2010 though, researchers continued to highlight other genes as candidates. See, for example, Wuebbles et al., Int. J. Clin. Exp. Pathol., 3(4): 386-400 (2010) highlighting the FSHD region gene 1 (frg1). In contrast, Wallace et al., Mol. Ther., 17(Suppl. 1): S151 (2009); Wei et al., Mol. Ther., 17(Suppl. 1): S200 (2009); and the Lemmers et al. report from the Sciencexpress issue of Aug. 19, 2010 highlight DUX4. Neguembor and Gabellini, Epigenomics, 2(2): 271-287 (2010) is a recent review article regarding FSHD.
RNA interference (RNAi) is a mechanism of gene regulation in eukaryotic cells that has been considered for the treatment of various diseases. RNAi refers to post-transcriptional control of gene expression mediated by microRNAs (miRNAs). The miRNAs are small (21-25 nucleotides), noncoding RNAs that share sequence homology and base-pair with 3′ untranslated regions of cognate messenger RNAs (mRNAs). The interaction between the miRNAs and mRNAs directs cellular gene silencing machinery to prevent the translation of the mRNAs. The RNAi pathway is summarized in Duan (Ed.), Section 7.3 of Chapter 7 in Muscle Gene Therapy, Springer Science+Business Media, LLC (2010).
As an understanding of natural RNAi pathways has developed, researchers have designed artificial miRNAs for use in regulating expression of target genes for treating disease. As described in Section 7.4 of Duan, supra, artificial miRNAs can be transcribed from DNA expression cassettes. The miRNA sequence specific for a target gene is transcribed along with sequences required to direct processing of the miRNA in a cell. Viral vectors such as adeno-associated virus have been used to deliver miRNAs to muscle [Fechner et al., J. Mol. Med., 86: 987-997 (2008)].
Adeno-associated virus (AAV) is a replication-deficient parvovirus, the single-stranded DNA genome of which is about 4.7 kb in length including 145 nucleotide inverted terminal repeat (ITRs). There are multiple serotypes of AAV. The nucleotide sequences of the genomes of the AAV serotypes are known. For example, the complete genome of AAV-1 is provided in GenBank Accession No. NC_002077; the complete genome of AAV-2 is provided in GenBank Accession No. NC_001401 and Srivastava et al., J. Virol., 45: 555-564 {1983); the complete genome of AAV-3 is provided in GenBank Accession No. NC_1829; the complete genome of AAV-4 is provided in GenBank Accession No. NC_001829; the AAV-5 genome is provided in GenBank Accession No. AF085716; the complete genome of AAV-6 is provided in GenBank Accession No. NC_00 1862; at least portions of AAV-7 and AAV-8 genomes are provided in GenBank Accession Nos. AX753246 and AX753249, respectively; the AAV-9 genome is provided in Gao et al., J. Virol., 78: 6381-6388 (2004); the AAV-10 genome is provided in Mol. Ther., 13(1): 67-76 (2006); and the AAV-11 genome is provided in Virology, 330(2): 375-383 (2004). Cis-acting sequences directing viral DNA replication (rep), encapsidation/packaging and host cell chromosome integration are contained within the AAV ITRs. Three AAV promoters (named p5, p19, and p40 for their relative map locations) drive the expression of the two AAV internal open reading frames encoding rep and cap genes. The two rep promoters (p5 and p19), coupled with the differential splicing of the single AAV intron (at nucleotides 2107 and 2227), result in the production of four rep proteins (rep 78, rep 68, rep 52, and rep 40) from the rep gene. Rep proteins possess multiple enzymatic properties that are ultimately responsible for replicating the viral genome. The cap gene is expressed from the p40 promoter and it encodes the three capsid proteins VP1, VP2, and VP3. Alternative splicing and non-consensus translational start sites are responsible for the production of the three related capsid proteins. A single consensus polyadenylation site is located at map position 95 of the AAV genome. The life cycle and genetics of AAV are reviewed in Muzyczka, Current Topics in Microbiology and Immunology, 158: 97-129 (1992).
AAV possesses unique features that make it attractive as a vector for delivering foreign DNA to cells, for example, in gene therapy. AAV infection of cells in culture is noncytopathic, and natural infection of humans and other animals is silent and asymptomatic. Moreover, AAV infects many mammalian cells allowing the possibility of targeting many different tissues in vivo. Moreover, AAV transduces slowly dividing and non-dividing cells, and can persist essentially for the lifetime of those cells as a transcriptionally active nuclear episome (extrachromosomal element). The AAV proviral genome is infectious as cloned DNA in plasmids which makes construction of recombinant genomes feasible. Furthermore, because the signals directing AAV replication, genome encapsidation and integration are contained within the ITRs of the AAV genome, some or all of the internal approximately 4.3 kb of the genome (encoding replication and structural capsid proteins, rep-cap) may be replaced with foreign DNA. The rep and cap proteins may be provided in trans. Another significant feature of AAV is that it is an extremely stable and hearty virus. It easily withstands the conditions used to inactivate adenovirus (56° to 65° C. for several hours), making cold preservation of AAV less critical. AAV may even be lyophilized. Finally, AAV-infected cells are not resistant to superinfection.
There remains a need in the art for a treatment for muscular dystrophies including FSHD.